my bios ver. is 1807 (check your ver. up here ASUSTeK Computer Inc. - Download -> Vista -> BIOS) if it's ok, may be you should try turning this device off completely (BIOS -> Onboard device configuration -> IEEE 1394 -> Disable (if i'm not mistaking) if you aren't using it then it'll be...
It works! I've got P5B-E and the problem highlighted above, and after checking the needed field in the "Power management" tab my computer shutdowned correctly. Thanks for the solution)